Support ClouderaManager Service Discovery in Admin UI

The Discovery Details part of the descriptor authoring / editing facilities in the Admin UI assume Ambari-based discovery. As such, there is currently no way to specify the discovery type.

The Discovery Details UI should include a drop-down based on the available supported discovery implementations. Ideally, this will be dynamically populated (via the service loader facility, or an API that depends on it?), rather than a static list.

It also should be possible to exclude any discovery configuration from the associated descriptor.
A no discovery option should be included in the drop-down list of discovery types.